1. Field of the Invention
The present invention relates to a method for treating a lithographic printing plate.
2. Description of the Related Art
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization or particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder, e.g., a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g., EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working preferred embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g., EP-625,728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer particle (latex), as described in e.g., EP-As 770 494, 770 495, 770 496 and 770 497. These patents disclose a method for making a lithographic printing plate including the steps of (1) image-wise exposing an imaging element including hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, (2) and developing the image-wise exposed element by applying fountain and/or ink.
Some of these thermal processes enable plate making without wet processing and are for example based on ablation of one or more layers of the coating. At the exposed areas the surface of an underlying layer is revealed which has a different affinity towards ink or fountain than the surface of the unexposed coating.
Other thermal processes which enable plate making without wet processing are for example processes based on a heat-induced hydrophilic/oleophilic conversion of one or more layers of the coating so that at exposed areas a different affinity towards ink or fountain is created than at the surface of the unexposed coating.
U.S. Pat. No. 4,576,743 discloses a plate cleaner including an aqueous solution containing a silicate and at least one surface active agent selected from a cationic or an amphoteric surface active agent.
EP 620 125 discloses a printing assistant for lithographic printing plates including a water-soluble hemicellulose compound derived from soyabean.
EP 1 356 949 provides a correction fluid for lithographic printing plates including a carbonic acid ester, a specified amide compound and compounds consisting of fluorocarboxylic acids, hydrogen fluoride alkaline compounds, fluoroalkaline compounds, and hydrofluoric acids of a metal belonging to Group 4 of Periodic Table.
EP 985 546 discloses a plate surface protective agent and a fountain solution composition for a lithographic printing plate each including a polyoxyethylene polyoxypropylene block copolymer.
Before, during and after the printing step, a lithographic printing plate is in general treated with various liquids for improving the lithographic properties of the image and non-image areas. Such liquids are applied for example to improve the hydrophilic properties of the non-image areas and to protect, restore or even enhance the hydrophobicity of the image areas. It is of high importance that these fluids, commonly referred to as plate treating liquids, do not deteriorate the image and/or the non-image areas throughout and well after their application. However, depending on the time of contact between the image areas and these chemicals, it has been observed that the image areas become thinner and that the aluminum substrate underneath even becomes visible. This disturbs the printed image in such a way that the press life—i.e., the number of acceptable printed copies that can be made with this plate—and/or the shelf life of the plate diminishes considerably after contact with such treating liquids. Especially the chemical resistance towards several treating liquids of the image areas of a printing plate based on a heat-induced solubility difference in an alkaline developer, is often insufficient. In addition, it is known in the art that the ink acceptance of the image areas—i.e., the so-called roll-up performance—is often reduced upon treating the plate with treating liquids of the prior art. Also, it has been observed that such a treatment drastically reduces the hydrophobicity of the image-areas, this reduction being known in the art as blinding. The reduction in roll-up performance and the blinding phenomenon are even more pronounced upon storage of the treated plate and as a result, the shelf life of the plate decreases.